1. CIT 470: Advanced Network and System Administration
Booting and Runlevels
CIT 470: Advanced Network and System Administration

2. CIT 470: Advanced Network and System Administration
Topics
Booting
Bootstrap loaders
Run levels
Startup scripts
Shutdown and reboot
CIT 470: Advanced Network and System Administration

3. CIT 470: Advanced Network and System Administration
Booting 4 3 2 1
ROM boot code (BIOS)
Bootloader
Kernel
Init
CIT 470: Advanced Network and System Administration

4. CIT 470: Advanced Network and System Administration1
BIOS
Load CMOS settings.
Initialize registers and power management.
POST (Power On Self Test.)‏
Display system settings (if key pressed.)‏
Activate other BIOSes
Disk interfaces
Graphics cards
Network interfaces
Find bootable device.
Load MBR.
Run MBR program.
CIT 470: Advanced Network and System Administration

5. CIT 470: Advanced Network and System Administration1
Dell BIOS Screenshot
CIT 470: Advanced Network and System Administration

6. MBR contains Bootloader2
MBR contains Bootloader
Only 446 bytes!
MBR bootloader bootstraps itself by loading larger bootloader program from disk.
CIT 470: Advanced Network and System Administration

7. CIT 470: Advanced Network and System Administration2
Bootloader: GRUB
CIT 470: Advanced Network and System Administration

8. CIT 470: Advanced Network and System Administration2
GRUB Boot Process
Stage 1 (stored in MBR or boot sector)‏
Detect the geometry and the accessing mode of the loading drive.
Finds and runs stage 1.5.
Stage 1.5 (stored at beginning of partition)
Understands filesystem; loads stage 2 from filesystem.
Stage 2 (stored in filesystem)‏
Displays menu of OS choices.
Often displays boot splashscreen.
Loads selected OS kernel.
fdisk /mbr will install the DOS MBR
NTLDR is the bootloader for MS Windows NT/XP.
CIT 470: Advanced Network and System Administration

9. CIT 470: Advanced Network and System Administration2
GRUB Installation
At OS Install Time
Most distributions install GRUB.
Linux Install
grub-install /dev/hda
Native install
Boot with CD and run grub.
#>> root (hd0,0)‏
#>> setup (hd0)‏
#>> quit
CIT 470: Advanced Network and System Administration

10. CIT 470: Advanced Network and System Administration2
GRUB Boot
Automatically boots default OS after timeout.
Key sequences bring up
Menu of boot options (described in menu.lst)‏
Edit kernel options (runlevel, other features)‏
GRUB command shell
A boot prompt password may be required before edit/shell access granted.
Can bypass OS security by specifying kernel options or accessing files from GRUB shell.
CIT 470: Advanced Network and System Administration

11. CIT 470: Advanced Network and System Administration2
Booting with GRUB
Direct Boot
Specify OS kernel image to load.
Chainloading
Specify another bootloader to load.
Chainload NTLDR to boot MS Windows.
CIT 470: Advanced Network and System Administration

12. Config: /boot/grub/menu.lst2
Config: /boot/grub/menu.lst
# timeout (sec) before booting default
timeout
title Ubuntu, kernel
root (hd1,0)‏
kernel /boot/vmlinuz root=/dev/hde1 ro quiet splash
initrd /boot/initrd.img
boot
title Windows 2000
root (hd0,0)‏
makeactive
chainloader +1
CIT 470: Advanced Network and System Administration

13. GRUB Naming Convention2
GRUB Naming Convention
GRUB uses BIOS hard disk numbers
OS may not number disks identically to BIOS.
Examples
(hd0,0) First partition of first hard disk
(hd1,2) Third partition of second hard disk.
CIT 470: Advanced Network and System Administration

14. CIT 470: Advanced Network and System Administration3
Kernel Booting
CIT 470: Advanced Network and System Administration

15. CIT 470: Advanced Network and System Administration3
Initial RAMDisk
Temporary filesystem used for booting.
Stored as /boot/initrd-VERSION
Loaded into memory by bootloader
Contains drivers needed to mount root fs
RAID, LVM, NFS or other device drivers
Filesystem drivers
Allows single kernel to be shipped by vendor
Drivers specific to your system on RAMdisk
Also supports hibernation for laptops
CIT 470: Advanced Network and System Administration

16. CIT 470: Advanced Network and System Administration3
Kernel Arguments
Numeric arguments
Specify runlevel.
single also specifies single user mode
Root device options
root= specifies which root device to use
ro, rw specify access type
Console options
console=ttyS1,9600 will use serial console
Hardware options
Enable, disable specific hardware devices/features.
CIT 470: Advanced Network and System Administration

17. CIT 470: Advanced Network and System Administration
Run Levels
0 – Shutdown
1 – Single user mode
2 – Multi-user, no networking, no GUI
3 – Multi-user, with networking, no GUI
4 – Unused
5 – Multi-user, networking, and GUI
6 – Reboot
Defined in /etc/inittab
CIT 470: Advanced Network and System Administration

18. CIT 470: Advanced Network and System Administration
Single User Mode
Single user mode has
No GUI.
No network access.
Only one user (root) can use system.
Why use single user mode?
Run fsck to fix filesystem corruption.
Backup files without filesystem active.
Fix problems without user interference.
May allow access to system without a password in case you have forgotten root’s password.
CIT 470: Advanced Network and System Administration

19. Booting in Single User Mode4
Booting in Single User Mode
At GRUB prompt, select ‘a’ to append args
Append this argument to list:
single
Hit return to complete argument changes
Select ‘b’ to boot the kernel
Single-user mode differs from run level 1 in that run level 1 runs the scripts for run level 1, then enters single-user mode.
CIT 470: Advanced Network and System Administration

20. What if you lost your root password?4
What if you lost your root password?
At GRUB prompt, select ‘a’ to append arguments
Append this argument to list:
init=/bin/bash
Hit return to complete argument changes
Select ‘b’ to boot the kernel
At root prompt, remount /
mount / -o rw,remount
Reset root password
passwd
Power cycle system
CIT 470: Advanced Network and System Administration

21. CIT 470: Advanced Network and System Administration
/etc/inittab
# The default runlevel.
id:2:initdefault:
# Boot-time system configuration/initialization script.
si::sysinit:/etc/init.d/rcS
# What to do in single-user mode.
~~:S:wait:/sbin/sulogin
# /etc/init.d executes the S and K scripts upon change
l0:0:wait:/etc/init.d/rc 0
l1:1:wait:/etc/init.d/rc 1
l2:2:wait:/etc/init.d/rc 2
l3:3:wait:/etc/init.d/rc 3
l4:4:wait:/etc/init.d/rc 4
l5:5:wait:/etc/init.d/rc 5
l6:6:wait:/etc/init.d/rc 6
# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
CIT 470: Advanced Network and System Administration

22. CIT 470: Advanced Network and System Administrationrc
Executed by init with runlevel as argument.
Starts and stops services for each runlevel.
/etc/rc#.d scripts
Symlinks to actual scripts in /etc/init.d
Stops services beginning with K*
Starts services beginning with S*
Executes in ASCII order.
CIT 470: Advanced Network and System Administration

23. CIT 470: Advanced Network and System Administration
Scripts in runlevel 2
$ ls /etc/rc2.d
S05vbesave S13gdm S20openntpd S89cron
S08iptables S18hplip S20postfix S91apache2
S10acpid S18portmap S20powernowd S98usplash
S10powernowd.early S19cupsys S20rsync S99acpi-support
S10sysklogd S20apmd S20ssh S99rc.local
S10syslog-ng S20argus-server S20xinetd S99rmnologin
S11klogd S20hotkey-setup S21nfs-common S99stop-readahead
S12dbus S20makedev S89anacron
CIT 470: Advanced Network and System Administration

24. CIT 470: Advanced Network and System Administration
Example init.d script
#! /bin/sh
case "$1" in 'start')
if [ -f /usr/local/sbin/sshd ]; then
echo "starting SSHD daemon" /usr/local/sbin/sshd & fi ;;
'stop')‏
PID=`/usr/bin/ps -e -u 0 | /usr/bin/fgrep sshd |
/usr/bin/awk '{print $1}'`
if [ ! -z "$PID" ] ; then
/usr/bin/kill ${PID} >/dev/null 2>&1 *)
echo "usage: /etc/init.d/sshd {start|stop}“
esac
CIT 470: Advanced Network and System Administration

25. Turning Services On/Off4
Turning Services On/Off
Add a new service
Install startup script in /etc/init.d
Create S symlinks in appropriate runlevels
Create K symlinks in appropriate runlevels
Prevent a service from starting on boot
Remove S links from /etc/rc?.d/*
Remove K links from /etc/rc?.d/*
CIT 470: Advanced Network and System Administration

26. Red Hat Service Commands4
Red Hat Service Commands
Starting or stopping a service
service NAME [start,restart,stop]
is equivalent to
/etc/init.d/NAME [start,restart,stop]
Enabling or disabling a service at boot time
chkconfig --level # NAME
ln –s /etc/init.d/NAME /etc/init.d/rc.d/rc#.d/S??name
CIT 470: Advanced Network and System Administration

27. Red Hat Service Configuration4
Red Hat Service Configuration
RH Linuxes use files in /etc/sysconfig
VARIABLE=value format
Documentation in RHEL manuals
Examples
desktop: select GNOME, KDE, XFCE desktop
iptables: firewall configuration
network-scripts/ifcfg-eth0: eth0 cfg

28. CIT 470: Advanced Network and System Administration
/etc/sysconfig
CIT 470: Advanced Network and System Administration

29. CIT 470: Advanced Network and System Administration
Shutdown
shutdown –h now (or init 0)‏
Offers time-delay option (-h time)
Runs server stop scripts.
Kill remaining processes.
Flushes writes to disk.
halt
Kills processes.
Flushes writes to disk (sync).
CIT 470: Advanced Network and System Administration

30. CIT 470: Advanced Network and System Administration
Reboot
reboot (or init 6)
restarts system
Ctrl-Alt-Del
Defined in /etc/inittab
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
Killing init will often shutdown or reboot system, but may cause kernel panic.
CIT 470: Advanced Network and System Administration

31. CIT 470: Advanced Network and System Administration
What's wrong with init?
Performance
Starts services sequentially.
Shell scripts spawn lots of processes.
Manual configuration
Order to start services determined manually.
i.e. network before authentication before nfs.
Reliability
Init doesn’t monitor or restart services after boot.
Hotplug hardware
Drives mounted at boot.
What about USB or network drives?
CIT 470: Advanced Network and System Administration

32. CIT 470: Advanced Network and System Administration
Init Alternatives
iniNG
Service config specifies dependencies.
need = system/initial net/all;
Starts services in parallel once dependencies met.
launchd
Mac replacement for init, rc, cron, atd, inetd
Monitors services, starts on demand.
Service Management Facility
Sun replacement for init with service monitoring.
CIT 470: Advanced Network and System Administration

33. CIT 470: Advanced Network and System Administration
Upstart
Event-based init replacement.
Events
Hardware device has been added.
Filesystem has been mounted.
Time (cron-replacement)
File has been modified.
Another job has begun or finished running.
Backwards compatible with /etc/init.d scripts.
Respawns services that are down.
CIT 470: Advanced Network and System Administration

34. CIT 470: Advanced Network and System Administration
Boot Process Summary
CIT 470: Advanced Network and System Administration

35. CIT 470: Advanced Network and System Administration
Key Points
Boot Steps
BIOS
Bootloader (GRUB)
Kernel
init
init is PID 1, parent of all processes, started by kernel
Uses rc command to run all scripts in /etc.rcN.d dir where N is runlevel
RHEL: use chkconfig to configure, /etc/sysconfig for options
Run levels
1 and Single user mode (use to fix problems)
3 = multiuser + network (servers)
5 = multiuser + network + GUI (workstations)
CIT 470: Advanced Network and System Administration

36. CIT 470: Advanced Network and System Administration
References
Aeleen Frisch, Essential System Administration, 3rd edition, O’Reilly, 2002.
M. Tim Jones, “Inside the Linux boot process,”
M. Tim Jones, “Parallelize applications for faster Linux booting,”
Evi Nemeth et al, UNIX System Administration Handbook, 3rd edition, Prentice Hall, 2001.
Scott James Remnant, “Upstart in Universe,”
Starman, “Boot records revealed: MBR details,”
“Extended boot record,”
GRUB manual,
“How it works: Master Boot Record (MBR),” atapi.com/hiwmbr.htm
“Replacement Init”,
CIT 470: Advanced Network and System Administration